1. Field of the Invention
This invention relates to processes for the preparation of Eu-doped BaFCl phosphors and more particularly to such processes in which the resulting phosphor particle size and particle size distribution is controllable.
2. Description of the Prior Art
There is much prior art in the phosphor field, but apparently none disclosing the process of the present invention. The most relevant art is considered to be as follows:
1. A. N. Campbell and A. J. R. Campbell, Trans. Faraday Soc. 35, 241 (1939), shows that BaFCl forms by precipitation from a water BaCl.sub.2 /BaF.sub.2 solution at 25.degree. C.
2. Fernberger, U.S. Pat. No. 2,373,071 discloses phosphors with divalent europium activators. Among the hosts cited are the alkaline earth fluorides CaF.sub.2 and SrF.sub.2. The patent states that "in preparing europium-activated phosphors, the general procedure may consist in adding a suitable europium compound to a batch of the matrix material in a fine state of division, and firing or heating the batch to a suitable temperature for a substantial length of time; the europium compound employed may be incorporated with the matrix material by dissolving the europium compound in water and thoroughly wetting down the finely divided matrix material with the solution to a stiff paste or a slurry, which may then be dried in a steam oven and powdered again. The bath may be heated with or without exposure to the air, or in a protective or reducing atmosphere, as of hydrogen; and sometimes different conditions may be employed in succession during the heating."
3. The copending, commonly assigned application of L. H. Brixner, U.S. Ser. No. 603,452 filed Aug. 11, 1975, and now abandoned, describes luminescent compositions comprising an organic binder and selected barium and strontium europium fluorohalides and the use of these compositions in luminescent screens.
4. The copending, commonly assigned application of A. Ferretti, U.S. Ser. No. 522,880 filed Nov. 11, 1974, now U.S. Pat. No. 3,988,252, discloses a process for making BaFCl:Eu phosphors by intimately mixing a substantially stoichiometric mixture of the appropriate metal halides, e.g., BaCl.sub.2, BaF.sub.2 and EuF.sub.3, having at least 30% of the particles less than 1.5.mu. with the remaining particles having an average particle size less than 3.mu., firing in an atmosphere free of hydrogen at 600.degree. to 900.degree. C. for a time sufficient to effect reaction. The product is then pulverized and the particles preferably classified to obtain a product with suitable particle size and particle size distribution for X-ray screens.
None of these references discloses the preparation of Eu-doped BaFCl phosphor of controllable particle size and particle size distribution by firing the host material BaFCl and a Eu source.